1. Field of the Invention
The present invention relates to a semiconductor integrated circuit having insulated gate field effect transistors (hereinafter called as IGFET's in plural case and as IGFET in singular case), and more particularly, to the integrated circuit having a structure and a circuit suitable for a high voltage operation.
2. Description of Related Art
In a conventional semiconductor integrated circuit combining, for high voltage operation, a signal level conversion circuit which is operated by multi-power supply voltages except the ground voltage, all the gate insulating films of the IGFET's used in the circuit have had the same thickness one another. Therefore, for operating the IGFET connected to the high voltage line side, a current path between the high voltage line and a ground line including a resistor must be provided so that a voltage drop induced in the resistor by allowing a current flow through the current path is applied to the gate electrode to render the IGFET in the conductive state. According to the circuit, the gate insulating film of the IGFET having a thin thickness as other IGFET's connected to the low voltage line side, is not fetally destroyed.
According to such conventional structure, however, the current must be kept to flow during the whole period in which the IGFET on the high voltage line side is in the conductive state. Therefore, the power consumption becomes inevitably large. To reduce the power consumption by minimizing the current, the resistance value of the resistor must be increased. However, in this case, the time for charging the gate capacitance of the IGFET on the high voltage line side is prolonged and the switching speed thereof is unfavorably delayed.